This invention relates to apparatus used for the analysis of the particles in an aerosol, and in particular measurement of particle mass concentration and size distribution of an aerosol.
Apparatus having piezoelectric crystal sensors have been used for a number of years to measure particle concentrations of aerosols. Such sensors are described in “The Performance of Piezo-Electric Crystal Sensors Used to Determine Aerosol Mass Concentration” by Daly S P and Lundgrun D A, American Industrial Hygiene Assoc. Journal, July, 1975, p518-532, Piezo-Electrostatic Aerosol Mass Concentration Monitor by Olin J G, Sem G J & Christenson D L, Amer. Ind. Hyg. Assoc. J. 32:209 (1970) and U.S. Pat. No. 3,653,253. Such apparatus include a piezoelectric crystal sensor which is exposed to a gas stream containing particles that deposit on the surface of the crystal. By measuring the frequency shift of the crystal and by knowing the mass sensitivity for the crystal, it is possible to determine the mass accumulated on the crystal surface. If the volume of the gas flow is known, the mass concentration can be determined.
A problem with the apparatus is that the crystal sensor can only function properly up to a maximum mass loading and, when used for environmental monitoring of particles, the sensor often becomes overloaded before the expiration of a standard test period; for example, a crystal sensor may overload in 1 or 2 days within a 14-day monitoring period for monitoring environmental particulates. Furthermore, the problems associated with the overloading of crystal sensors reduces the usefulness of such sensors for testing vehicle emissions.
The present invention provides improved apparatus which is useful for continuous monitoring of particulate concentrations, and which is also useful for the analysis of collected particles.